Clean technology series: long-duration energy storage

As the global transition to net zero accelerates, the generation of renewable energy is expanding at an unprecedented rate. Solar and wind are now among the cheapest sources of electricity available, and their deployment continues to gather momentum worldwide. However, as we discussed previously, both solar and wind power come with a fundamental challenge: intermittency. The sun does not always shine, and the wind does not always blow.

Other forms of renewable energy generation such as hydroelectric and geothermal can provide reliable, weather-independent electricity.  Yet even with these technologies playing an increasing role in the energy mix, a key challenge remains: how can large quantities of renewable energy be stored and deployed when needed?

In our recent article on geothermal energy, we touched on the possibility that underground geothermal reservoirs could themselves function as large-scale long duration energy stores.  In this latest instalment of our clean technology series, we examine the emerging world of long duration energy storage (LDES), a diverse collection of technologies designed to store energy for days, weeks, or even longer. As power systems become increasingly reliant on renewable energy, LDES may prove to be one of the most important enablers of the transition away from fossil fuels.

Beyond lithium-ion: why long-duration storage matters

Lithium-ion batteries have become synonymous with energy storage. They have helped integrate renewable energy generation into power systems by smoothing grid fluctuations and have accelerated the adoption of electric vehicles.

However, most grid-scale lithium-ion systems are optimised for relatively short discharge durations, typically measured in hours rather than days. They excel at balancing short-term fluctuations in supply and demand, for example by storing solar power during the day and releasing it at night, but lithium-ion batteries were never designed to store huge quantities of energy for extended periods.

As renewable energy generation increases, power systems will require storage solutions capable of bridging potentially long gaps in generation. These gaps could arise during periods of low wind across an entire region, extended cloudy conditions, or seasonal fluctuations. The challenge is no longer simply generating clean electricity, but storing vast quantities of generated energy economically and reliably.

Thermal energy storage

Perhaps surprisingly, some of the most promising long duration energy storage technologies rely not on advanced battery chemistries but on the storage of heat.  During periods of abundant renewable generation, electrical heating elements can be used to raise the temperature of materials such as bricks, concrete, rocks, sand, or molten salts. The thermal energy can then be retained in the material for days before being used directly in industrial processes or converted back into electricity when needed.

One fairly well developed approach employs molten salts as a heat storage medium, allowing renewable electricity to be converted into high-temperature industrial steam on demand using the same technology underpinning concentrated solar power plants. Other approaches employ brick-like thermal storage blocks capable of storing large quantities of thermal energy (at temperatures exceeding 1,000°C) for use in generating hot air or steam [1].

Sand and rocks are abundant, inexpensive, non-toxic, and free from many of the supply chain concerns associated with battery materials. Moreover, around half of global energy consumption ultimately takes the form of heat, particularly in industrial sectors such as chemicals, steel, food processing, and cement. In such industrial applications, converting electricity directly into stored heat can be a highly efficient form of energy storage.

Gravitational storage: what goes up must come down

Another technology attracting significant attention takes advantage of a well-understood concept: gravity.

Traditional pumped hydroelectric storage already represents the world’s largest deployed energy storage technology. During periods of low electricity demand, water is pumped uphill into a reservoir and subsequently released downhill through turbines when power is required.

A variation on pumped hydro storage involves replacing water with another working fluid, such as a high-density slurry having a density greater than that of water [2]. By increasing the density of the fluid, energy storage equivalent to that of conventional pumped hydro can be achieved using significantly smaller reservoirs or lower elevation differences. This has the potential to expand the range of suitable deployment locations dramatically, opening up sites that would previously have been considered uneconomic or geographically unsuitable.

The principles of gravitational energy storage can also be applied beyond fluid-based systems.  One particularly interesting approach repurposes abandoned mine shafts as vertical energy stores.  In these systems, surplus renewable electricity is used to raise heavy weights within deep shafts. When electricity is required, the weights are lowered, driving generators and returning power to the grid. Patent activity in this area includes multi-weight storage systems designed to maximise the amount of energy that can be stored within a single shaft while providing a more continuous power output [3].

Unlike electrochemical batteries, gravitational storage systems generally avoid degradation associated with repeated charge-discharge cycles and can often utilise existing infrastructure.  Although the underlying principles are well understood, innovative engineering can expand the practical reach of gravity-based storage.

Geothermal reservoirs

One of the most intriguing long duration storage concepts emerges naturally from the next-generation geothermal technologies discussed in our previous article.

Research suggests that enhanced geothermal systems (EGS) may function not only as sources of reliable clean electricity, but also as large-scale energy storage assets [4].  In much the same way that some gravity-based storage concepts seek to repurpose existing reservoirs and mines, this approach aims to unlock a second function from geothermal infrastructure already developed for energy generation.  Importantly, because the storage capability arises from infrastructure already required for power generation, the additional storage functionality may be achieved at comparatively low cost.

The concept is deceptively simple.  During periods of abundant renewable generation, excess electrical energy is used to inject water into an engineered geothermal reservoir, increasing pressure within the subsurface fracture network. Because the surrounding rock acts as a natural seal, the pressure can be retained for extended periods with minimal losses.  When demand increases, operators can release the stored pressure by adjusting production wells. The accelerated flow of hot water and steam can then drive turbines and generate electricity on demand.

This dual role as both a source of energy generation and a means of energy storage could fundamentally change the economics of geothermal projects and further enhance their value within renewable-heavy electricity systems.

Protecting innovation in long duration storage

Although many LDES technologies appear mechanically straightforward and rely on long-standing and well-understood scientific principles, the innovation landscape is increasingly sophisticated and covers a wide range of areas, including:

• Advanced thermal storage media and containment systems;
• High-temperature insulation materials;
• Heat exchanger configurations and architectures;
• Gravity storage mechanisms and control systems;
• Geothermal pressure management technologies;
• Digital optimisation platforms and predictive control algorithms; and
• Grid integration and energy management software.

The growing importance of long duration energy storage and the role of innovation in moving that forward has also been recognised by policymakers. In August 2026, the UK Government launched its £28 million Ultra-Long Duration Energy Storage (Ultra-LDES) Challenge, aimed at accelerating the development of technologies capable of storing clean electricity for more than 100 hours [5]. Offering grants to support the development of new technology, the initiative reflects an increasing recognition that achieving a secure, renewable-powered energy system will require not only clean generation, but also the ability to store energy over timescales measured in days.

Looking ahead

The clean energy transition has often been framed as a generation challenge. However, as the deployment of renewable energy sources continues to accelerate, storage is rapidly becoming just as important. As a result, long duration energy storage may become one of the defining clean technology sectors of the coming decade.

No single technology is likely to dominate the LDES landscape. Instead, future power systems will probably employ a mix of complementary solutions. Lithium-ion batteries may continue to provide rapid, short-duration balancing. Thermal systems could decarbonise industrial heat. Gravity storage may exploit local geographical opportunities. Geothermal reservoirs could combine renewable energy generation with flexible energy storage.

From heated bricks and molten salts to mine shafts, pressure reservoirs and gravity itself, some of the most transformative long duration energy storage solutions may rely not on exotic materials or revolutionary chemistry, but on the intelligent application of heat, gravity, pressure and other remarkably simple physical principles.

For more information or advice on protecting your invention and the help that is available, please get in touch with Alexandra Collingwood-Pierce or your usual Venner Shipley adviser.